You did not say if you used the matching balancer and flywheel or flex plate. I guess its up to you to pull and check out if you did . Where did the kit come from? You have the bob weight card?

 

When I brought the spinning assembly,( pistons, rods, crank, flexplate, balancer, rings & bearings) to the machine shop, they were supposed to be balanced together. external balanced crank, using 400 balancer & flexplate.

 

Something did not get balanced or you got the wrong balancer or flexplate. Need to start over. I don't know where to start but I would guess the balancer and flex are in question. Back to the shop that did all this. If they won't help you, its your baby now.

 

73, ( Name ???? )

I could be wrong, but if the vibration goes away above 2100 rpm, I am going to bet that it is not a balance problem. An out of balance condition would get a lot worse as the rpms increase. An out of balance tire gets worse the faster you spin it, same with a crankshaft.

You did not mention your combination. If you have a fairly radical camshaft, it might just have a "bad" spot in the rpm range that causes a lot of shaking. The cam I had in my 68 Camaro race car was like that, used to rattle all of the sheet metal in the car coming down the return road but would turn 7800 rpm smooth as could be.

The other thing that I have seen cause strange "vibrations" is the clutch fan. Try taking the belts off the engine and run it for a couple of minutes to see if it is better. Double check the firning order too, I have seen two plug wires crossed cause a pretty wierd "miss" that went away at higher rpm.

Just to be sure, you might want to see if you can find someone familiar with the way race engines run to give you a second opinion before you start tearing things apart.

Just on a chance that it is really a balance problem .... did the crank have a dowel pin in the back to make sure the flywheel got installed in the correct position ? Also, what type of damper is it ?

 

definitely the right balancer & dampener, 400 parts are hard to miss. the balncer is steel ring type,(with the "cut out"), but chromed, flexplate is HD with the weight on it. Cam is Extreme Energy 224/234 if I remember correctly. flat tops with 76 cc heads, matching springs & lifters. performer manifold with 750 Comp AFB carb. Hei mallory dist, Headers into 2 1/2" exaust. the flex plate IS installed correctly. I've been looking at THAT Fan. will disconnect it tomorrow.

 

I would pull the belt first. Then try a different harmonic balancer. What about motor/tranny mounts and body mounts. Are you running solid motor mounts and aluminum body bushings? Rule out everything before you pull it apart.

 

Forgive me if I'm wrong on this, but wouldn't changing the harmonic balancer require a total rebalance of the engine?

I know when I had my 383 balanced, the brand new harmonic balancer I gave them had new holes drilled into the back of it. I would assume changing the balancer would mess this up.


I would agree with what others have said. Disconnect all of the belts for a few minutes, to make sure the accessories / fan aren't causing it.
Next disconnect the exhaust and see what happens.

If all of this still doesn't fix it, changing camshafts might be the last thing to try.

After that I think it's pretty safe to say something is wrong internally and you are going to need to rebuild again.

 

All new rubber motor and trans mounts. All neww rubber Body to frame mounts.New sub frame connectors,(but it was doing it before I installed the sub frame connectors.

 

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I got one doing this now that I'm chasing. It'll rattle your teeth out but as rpm comes up it smooths out. I unhooked the convertor and it has an electric fan. It has a semi-radical cam (288/296 252/258 630/630 106/102) and Bill has me thinking. I done chased electrical, tune, pulled the oil pan, etc. I got a new TCI flexplate and Pioneer balancer. It idles at 1200, is pretty smooth coming up on the throttle, then hits a rough spot, then smoothes back out about 4000. If I find something, I'll post it.

 

My 496 does the same thing but at a different rpm. With the exhaust installed @2600-2800 has a vibration but with open hedders no vibration. Maybe some type of reversion pulse

 

good point, the front end on my truck shakes at 60 but not at 70

 

when i bought my 68 377, 4 spd, it had a vibration so bad you couldn't hold on to the shifter long , it felt like the bones in your hand were coming apart. not to mention the car. after many hours of looking for the problem, i found two broken ear om the m21, flywheel shot, wrong clutch, and a banged up driveshaft. just giving you some more options, even though you have an automatic.

 

already pulled the belts, no change. disconnected the converter, no change. They did not drill any holes in the balancer. ONLY THE CRANK. Matter of fact, when I got the balancer (which is chrome) & the new anodized flexplate back,there were no scratches on the mounting surface , OR on the flywheel where the bolts hold it to the crank showing that EITHER of them had ever been even Bolted On ! I wondered about that at the time.

 

Any chance you have a flexplate for a 454 instead of for a 400? I believe they are weighted differently.

 

i had the same problem except at idle it also wanted to surge forward i never found out exactly what it was but after playing with the timing and carb tuning 90% of it is gone my symtoms were engine shaking harshly at idle an cruising, hrd to turn off and car wanting to move forward while my foot was on the brake but ran hrd on the freeway hope this helps alittle

 

Ok I'm gonna throw this out there, see what you guys think.


What if the shop thought it was an "internally balanced" 383, so they only balanced the internal parts (crank, pistons,etc). This would mean no balancing done on the harmonic balancer / fllexplate.


This would A, explain the problem, and B explain why some of his parts had no drill holes.



P.S. I know for a fact that my balancer had new holes in it after they were done, I dunno if EVERY balance includes drilling there, but mine did.


EDIT: I just read that after looking at your parts they didn't even bolt them to the crank. This definately tells me they balanced it wrong, I would call them tomorrow. One would assume you could balance it by just taking weight off the crank, but impossible to get a correct balance when not all of the parts are being taken into account.

 

The dampner and flexplate are supposed to be a set weight. All balancing should be done on the crank, rods, and pistons. I would have got pretty upset if my machine shop would have drilled my balancer instead of my counter weights. It would suck if it ever slips a ring or needs replacing.

 

73 Stroker -"Post the bob weight sheet so we can make sure nothing got messed up." Not tying to point any fingers just thought it could not hurt to double check.
Sound stupid, but when i worked at a engine shop i balanced a ton of engines from 1974-1980 and 1 late nite into a 18 hour day, i messed up and put 2 connecting rod small end weights on the bob weight total. Added about 200 grams to the bob weight figure if i recall correctly. That thing shook at low rpm, but you could almost drive through it like you are describing. We used to modify the balancer and flywheel on balance jobs back then, if that is what it took to get the job done, now i only do the modifications to the crank and use the stock weights on the balancer and flywheel. I now use heavy metal if needed, in the crank, to make any changes, or drill the crank counterweights. The fix at that time was to put a neutral balanced flywheel and balancer on the engine, as i had modified the existing balancer and flywheel. Kinda makes you wonder how much twisting and bending the internal componenets recieve from the shaking.
Hope this helps somewhat.
Mike.

 

As stupid as this sounds, they vibrate bad when a flex plate starts to crack at the crank flange........ A friend had to replace at least two over the years, 350/TH400...

Fwiw,I had my 496 Detroit balanced and when I got it back, my new OEM damper and aftermarket flywheel both had been installed and had no other marks/holes in them!!

I messed up b/c I missed the fact that the guy told me to install at least .25" longer rods b/c it would have probably balanced with no heavy-metal. I used my good truck rods so ended up having to get them to cam-cut the crank so the rod pin bosses would clear the counterweights at the bottom of the stroke which then required heavy-metal to re-balance it..

Live and learn, no biggie b/c back then the difference between longer rods and heavy-metal balancing it was a cost wash!!

pdq67

PS., I only did it this way b/c I wanted to use my truck rods as well as I AM NOT going to rpm her much above 5500 rpm b/c I built her for max. midrange grunt NOT topend hp..

 

Couple of things I had vibration problems with in the past.
1) Bad crome crank pulley. Had a very slight wobble to it giving out a vibration around the rpm's you mentioned.
2) Bad bearing in waterpump.

Good luck!

 

Hmm thanks for clearing that up camaro_fever.

I guess next time I'm in the garage I will take a closer look, I'm kinda second guessing whether they did drill it or not.

 

I had same "shake" issue with 454 engine. Exactly the same shake as you describe in the very first post. I did all things to test, same as you; disconnect belts, removed tranny and ran with no clutch, etc, etc.

Anyway, took engine out and apart, sent back to balance shop. They re-balanced, and admitted it was bad balance job. So, I re-install, and guess what?? Still shaking......... Too much shake to drive. Really aggravated me, so I take engine out AGAIN and this time I go back to balance shop to actually watch bobweight calculation, parts matching, etc. The guy was confident that the 2nd balance would be spot-on and I could tell he thought I was wasting his time, UNTIL he started to spin the assembly and it just about shook off the balancer. I mean, it needed some serious work to correct. I watched the whole process, from start to finish. Bottom line, the 3rd time was a charm and it's smooth now. But this whole process drove me crazy..........

Thing is; it's a top-rated machine shop. They build a ton of race engines. So, the moral of the story is: even the best guys screw up sometimes.

Sounds to me like you need to dis-assemble engine and get it re-balanced.

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